This invention relates to a rupture testing apparatus for boiler tubes and more particularly to such an apparatus for determining the approximate remaining life of a boiler tube by subjecting it to a hoop stress rupture test.
Two basic methods have been used to date to estimate the remaining life of a tube subjected to internal pressure, such as a boiler tube that carriers water and/or steam at relatively high pressures. The first method utilizes sections of a representative tube subjected to operating pressure and elevated temperatures to accelerate the creep stress to which the tube is subjected. However due to the length required for the test specimens the tubes may have to be removed from an area which had not been exposed to the same degradation of properties if the boiler section in question had experienced a very localized area of distress. Also, due to boiler design considerations, i.e. bends, welds, brackets, etc., a sufficient sample tube length may not be available. Also it is difficult to run multiple tests at high pressures and temperatures rendering the tests more expensive. Further, the metallurgical samples may have to be taken a distance from the critical area in question and may not be representative of this area.
A second method of estimating the remaining life of a tube involves the use of miniature cordal tensile specimens machined from thick wall tubes. However, this is a uniaxial test and since most tube damage initiates near the surface the initial damage may be machined away in preparing this type of specimen. Also, since the relatively small cross sectional area and newly machined surfaces are exposed to atmosphere, the resulting higher oxidation rates could have a significant affect on the test results. Further, the specimens cannot be manufactured from thin wall tubes.